Earth Day at 40: Our Changing View of the Planet

Those iconic images of the Earth rising above the moon and the blue of the oceans against the blackness of space may seem like old hat nowadays – a view of Earth almost everyone recognizes.

But these whole-globe photos have only been available since the dawn of the space age, just before the birth of Earth Day, when satellites, spacecraft and astronauts have changed mankind's view of our home planet, showing us the beauty and fragility of the Earth in images taken from space.

"Over the last 50 years, satellites have revolutionized the way we see Earth," said Holli Riebeek, an education and public outreach coordinator for NASA's Terra satellite mission. "For the first time, we can see Earth as a planet, study Earth as a planet."

In ancient times, humanity's view of the Earth was rather limited. Some sages thought the world was flat, or filled with dragons and sea monsters in unexplored swaths of the oceans. It wasn't until intrepid seafarers began crisscrossing the oceans in search of new worlds and riches that most people accepted the planet was round.

But in 1968, just a year before Earth Day began and humans set foot on the moon, an iconic photo taken by Apollo 8 astronauts revealed humanity's home planet like never before – a fragile island of life hovering alone in space. [See Earth from space.]

Since then, bigger satellites, space probes and better cameras for astronauts have revealed Earth in crystal clarity.

Here, SPACE.com surveys some of the major milestones in Earth observation from space:

1958 – Explorer 1: The first American satellite launched into orbit, Explorer 1, gave scientists measurements of Earth's electrically charged ionosphere and protective magnetic field, though it didn't take any photographs of the Earth's surface.

1960 – TIROS-1: The world's first weather satellite, it was launched 50 years ago. While TIROS-1's images of the Earth and its weather systems were crude by today's standards, they gave scientists their first view of storm systems on a large scaele. In its first black-and-white image, TIROS-1 saw a fuzzy band of clouds over the United States. "The early TIROS satellites were basically a television camera that relayed images to ground stations on Earth," Riebeek told SPACE.com.

1968 – Apollo 8: The first astronauts ever to see the entire planet as a distant orb in a sea of black space were the three Apollo 8 astronauts, who took the iconic image of Earth rising over the limb of the moon in December 1968.

1990 – Voyager 1: Launched on Sept. 5, 1977, Voyager 1 was sent to get close-up views of the planets of the outer solar system. In 1990, having completed its primary mission, the spacecraft turned around to take a "family portrait" of all the planets in the solar system. On Feb. 14 of that year, it snapped the famous "pale blue dot" image of Earth at a distance of more than 4 billion miles (6.4 billion kilometers) from the planet that launched it.

2003 – Mars Global Surveyor: This Mars orbiter was launched to investigate our ruddy neighbor, but in 2003, it pointed its camera homeward and captured a surprisingly detailed image of our blue world suspended in the vast black of space. It was the first image of Earth taken from another planet in which the continents and clouds of the Earth were visible.

2004 – Spirit: After arriving on Mars, the Mars Exploration Rover Spirit looked up into the Martian sky and snapped a picture of Earth -- the first ever taken from the surface of another planet. The photo showed Earth as a bright dot above the horizon about an hour before sunrise.

2007 – Kaguya (SELENE): This Japanese moon probe replicated the famous Apollo-era "Earth-rise" photograph with modern high-definition imaging. Kaguya took a series of still images that show the Earth setting below the lunar horizon.

2010 – The International Space Station: After more than a decade of construction by 16 different countries, the $100 billion space station finally received what some astronauts consider its crown jewel – a seven-window observation deck that offers panoramic views of Earth from 220 miles up. Called the Cupola, the deck includes the largest space window ever built and is a favorite haunt for astronauts hoping to look down at their home planet.

These of course are just a bare sampling of the images of Earth as a whole and close-ups of its features that have been taken since humans first launched into space. The Earth is literally surrounded by weather and other observation satellites keeping close watch on the planet's air, land and seas.

May Earth-orbiting satellites have allowed scientists to see whole landforms – such as rivers, forests and deserts ? and connect Earth's interacting systems from weather to earthquakes in a way that wasn't possible before we could take a bird's-eye view of our planet.

And since the first generation of satellites was launched, each subsequent generation has made improvements on Earth-observing capabilities with higher resolution images and expanded sensing capabilities. For example, this image compares an early TIROS view of the Earth with one of the same area taken by Terra's MODIS sensor.

New satellites are even starting to take 3-D views of the planet.

NASA continues to build and launch satellite and spacecraft that will obtain more and better images of the Earth, including examples of how humans have altered the Earth with satellites dedicated to monitoring climate change and the melting of the planet's ice.

• Top 10 Views of Earth from Space
• Images: Earth from Above
• How Weather Satellites Changed the World